The present invention relates generally to chemical detection and more particularly to an apparatus and process for the measurement of hydrogen, a flammable material, when the hydrogen is being transferred from one storage container to another storage container.
Based on the literature and general good practices to those skilled in the art of transferring toxic and/or flammable material from one storage container to another container, only external “leak” detection devices have been used, i.e., gas detectors that alert the proper personnel once the flammable material has “escaped” to the open environment from the supposedly-closed system. For example, when natural gas is transferred, usually a mercaptan (e.g., tertiary butyl mercaptan, isopropyl mercaptan, normal propyl mercaptan, dimethyl sulfide mercaptan, and methyl ethyl sulfide) is combined with the natural gas flow simply to provide an odor that can be detected by a person in the vicinity of the natural gas leak to alert that person that a leak is occurring.
For the past several years, where hydrogen is the flammable material being transferred, gas detector devices available on the market have utilized thermal conductivity technology, electrochemical technologies, metal-oxide semiconductor (MOS) technologies, or optical technologies all of which suffer from the disadvantages discussed below. For example, thermal conductivity sensors may pose an “ignition source” problem that can ignite leaking hydrogen, when the hydrogen concentration is greater than about 4% per volume; electrochemical and MOS technologies require the presence of oxygen to operate; optical sensors cannot get wet or be exposed to a wet environment and therefore must typically be used in a clean and dry environment.
One type of conventional device used as a flammable gas detector is the combustible gas indicator (CGI) such as that sold by Mine Safety Appliances Co. of Pittsburgh, Pa., as well as other safety device manufacturers. The CGI is one of the most widely used instruments to provide a warning to safety responders when flammable substances in the atmosphere begin to approach their explosive limits. Most fire departments and industrial facilities have such instruments. The CGI is a non-specific detector that detects flammable gases in the atmosphere. Its operation is based upon the catalytic combustion of the flammable gas on a filament in a detector known as a “Wheatstone Bridge”. The CGI is calibrated with a flammable gas (e.g., hexane) using a known concentration referenced to NIST (National Institute of Standard Technology). The burning of this known concentration of the calibrant gas on the filament (relative to a reference “cool” filament) produces a signal, which is directly proportional to this specified concentration of the calibrant gas. In the field, the detection of a different flammable gas produces a signal that can be related to the response of the calibrant gas by pre-determined “response factors” that are provided by the manufacturer of the instrument. However, as mentioned earlier, this approach cannot be used within the fueling nozzle because of its mode of operation, because its hot wire-filament, can cause an unsafe condition, by generating a spark source.
For hydrogen, the flammable detection means requires substantial amounts of oxygen (e.g., >10%) be present, best applied to metal oxide semiconductor (MOS) sensor technology. This is also not acceptable because mixing air, or oxygen, with hydrogen (i.e., 4%–74% hydrogen in air or 4%–90% hydrogen in oxygen) in the presence of an ignition source (e.g., at least 0.02 millijoules), results in a dangerous condition. Currently, MOS-based sensors as primary information providers are widely used in many fields of technology and industry for environmental analysis. The most ardent problems of MOS-based sensor manufacturing are the reproduction of resistive properties and possibility of the formation of thin film metal oxide sensitive layers in certain configurations.
Another commercial means of flammable gas detection involves use of a catalytic bead sensor (which also requires the presence of oxygen). This type of sensor is made from two separate elements or “beads” that surround a wire operating at a high temperature (approximately 450° C.). A first element, the active element, is made by winding a small coil of wire, sealing it in a ceramic substance, and then coating it with a catalyst to promote a reaction with the gas. The second element, the reference element, is made identical to the active element except in place of the catalyst, a passivating substance is used to prevent this bead from reacting with the gas molecules. The reference bead compensates for changes in ambient temperature, humidity, and pressure variations. The beads are generally placed in separate legs of a Wheatstone bridge circuit. In theory, when gas comes into the environment, it has no effect on the passivated bead, but has a significant effect (primarily in terms of temperature) on the catalyzed bead. The increase in heat increases the resistance; the difference between the readings of the wires in the two beads forms the sensor signal. However, catalytic bead sensors operate above a threshold or “turn-on” voltage corresponding to the bead temperature that can, in the presence of the catalyst and oxygen, first ignite the gas. As the sensor ages, the catalyst slowly deactivates on the bead. The threshold voltage gradually increases, and the sensor sensitivity decreases. At the same time, changes in the wire coil cause increased zero drift and noise. The result is the sensor must be replaced. When a mixture of combustible gas or vapor in air diffuses through the sensor flame arrestor, it oxidizes on the catalytically-treated sensing bead. Since this oxidation reaction is exothermic, it causes an increase in the temperature of this bead (in relationship to the temperature of the reference bead) and a resulting increase in the electrical resistance of a small platinum coil embedded in this bead. The change in resistance in the embedded platinum coil is proportional to the amount of chemical energy released by the oxidation reaction. Electronic circuitry (e.g., a transmitter) immediately detects this increase in resistance and reduces electrical power to the bead until the original platinum coil resistance is restored. The amount of electrical power removed is linearly proportional to the combustible gas concentration present.
Electrochemical sensors utilize a technology similar to fuel cells. Fuel cells consist of an electrolyte with an anode on one side and a cathode on the other. They create electricity by passing a gas (usually hydrogen) over the anode and oxygen over the cathode. The two electrodes are separated by an electrolyte. This produces electricity, water, and heat. Electrochemical sensors work the same way. The gas passing over the electrode creates a chemical reaction and electrical current. The current generated is proportional to the amount of gas in the cell. In order for these to work, there must be oxygen on the other side of the cell.
Various gas sensor configurations are shown in U.S. Pat. No. 5,279,795 (Hughes et al.); U.S. Pat. No. 6,293,137 (Liu et al.); U.S. Pat. No. 5,012,672 (McKee); U.S. Pat. No. 4,782,302 (Bastasz), U.S. Pat. No. 5,834,627 (Ricco et al.); and U.S. Pat. No. 5,932,797 (Myneni).
There remains a need for an apparatus/method that provides for the measurement of hydrogen levels when transferring hydrogen from one container to another while utilizing a hydrogen sensor that is selective only to hydrogen and does not cross interfere with other species, does not rely on temperature differentials (e.g., thermal conductive sensors such as catalytic bead sensors), oxygen (e.g., electrochemical and MOS sensors) and does not require a clean and dry environment (e.g., optical sensors) and which can be positioned inside a nozzle transferring the hydrogen without saturating the sensor.